Oxygen is a gas having a high industrial interest because it has many applications in very varied technical fields: the production of steel, glass or paper, medicine, metal welding, combustion or depollution, for example.
One of the techniques used at present to produce oxygen is the so-called "PSA" (Pressure Swing Adsorption) technique. In the field of the invention there is meant by PSA processes, not only the PSA processes properly so-called, but also analogous processes, such as VSA (Vacuum Swing Adsorption) or MPSA (Mixed Pressure Swing Adsorption) processes.
According to this PSA technique, the oxygen contained in a gaseous mixture comprising essentially oxygen and nitrogen, such as air, is separated from the gaseous mixture by adsorption of the nitrogen on a material preferentially adsorbing nitrogen, said adsorption of nitrogen being carried out by pressure variation applied in the separation zone containing said adsorbent material; the oxygen which is not or only little adsorbed, is recovered at the outlet of said separation zone.
Such PSA processes have already been described often in the prior art. Schematically, a PSA process always comprises:
a selective adsorption step of the nitrogen on an adsorbent material, at a so-called "high pressure" of adsorption; PA1 a desorption step of the nitrogen trapped by the adsorbent, at a desorption pressure below the adsorption pressure, called a "low pressure"; PA1 a repressurization step of the separation zone comprising the adsorbent, by passage from the low pressure to the high pressure; and the oxygen product being recovered during the desorption phase of the nitrogen. PA1 a supply phase comprising an introduction of the gaseous flow at a supply temperature (T.sub.sup) into the separation zone with passage from the low desorption pressure to the high adsorption pressure, PA1 a purge phase comprising a desorption of the nitrogen adsorbed on said adsorbent material at a low desorption pressure below said high adsorption pressure, PA1 characterized in that it comprises moreover a regulation of the supply temperature (T.sub.sup) of the gaseous flow, such as air, to be separated and an adjustment of the high adsorption pressure. PA1 adjustment of the high adsorption pressure is carried out by introduction, into the supply phase, of a dead time of variable duration X, PA1 the high pressure is maintained substantially equal to a predetermined reference pressure value, PA1 the supply temperature T.sub.sup is maintained substantially equal to a predetermined reference temperature value, PA1 an initial dead time of a duration Xo is selected such that: ##EQU1## in which: d is the duration of supply of the separation zone with gaseous flow to be separated, PA1 T.sub.sup max is the maximum supply temperature (in K) of the gas flow at which the PSA unit is to be operated on the site in question, PA1 and T.sub.sup o is the mean supply temperature (in K) of the gas flow on the site in question, PA1 the high adsorption pressure is comprised between 10.sup.5 Pa and 10.sup.6 Pa, preferably of the order of 1.4.times.10.sup.5 Pa, PA1 the supply temperature (T.sub.sup) is comprised between 10.degree. C. and 60.degree. C., preferably between 25.degree. C. and 45.degree. C., PA1 the maximum supply temperature (T.sub.sup max) is comprised between 288K and 333K, PA1 the mean supply temperature (T.sub.sup o) is comprised between 273K and 303K, PA1 the duration (d) of supply is below or equal to 45 seconds, PA1 the gas flow to be separated is air, PA1 the adsorbent material is selected from zeolites of type X or A, and preferably said zeolite comprises at least 50% of AlO.sub.2 associated with cations selected from the group consisting of the cations calcium, lithium, zinc, copper, manganese, magnesium, nickel or any alkali or alkaline earth metal.
From this, it will be easy to see that the efficiency of separation of the gaseous mixture depends on numerous parameters, such as the high pressure, the low pressure, the type of adsorbent material and the affinity of the latter for the compounds to be separated, the composition of the gaseous mixture to be separated, the adsorption temperature of the mixture to be separated, the size and shape of the adsorbent balls, the composition of these balls and the temperature gradient established within the bed of adsorbent, for example.
Until now, no law of general behavior has however been able to be determined, because it is very difficult to connect the different parameters to each other.
Thus, U.S. Pat. No. 3,140,933 discloses a PSA process using a type X zeolite exchanged with lithium, but does not indicate either the supply temperature or within which preferred ranges of adsorption pressure (high pressure) and desorption pressure (low pressure) it is desirable to work.
Similarly, EP-A-0667183 discloses a PSA process using a type X zeolite exchanged 50-95% with lithium cations, 4-50% with trivalent cations. There again, no preferred range of supply temperature, of adsorption pressure or of desorption pressure is indicated.
There exist on the other hand references more or less contradictory having regard to the temperature parameter.
Thus, U.S. Pat. No. 3,973,931 discloses a PSA process in which the temperature variations within the adsorbent bed are accentuated by heating of said bed by means of an external heat source.
Conversely, U.S. Pat. No. 5,169,413 discloses a PSA process, in which the bed of adsorbent is cooled to a temperature below the ambient temperature by means of an internal refrigeration system. The teaching of this document is hence contradictory to that of the preceding one.
Moreover, other documents emphasize the necessity of proceeding with a regulation of the PSA process over time.
Thus, U.S. Pat. No. 5,529,607 discloses a PSA process comprising at least two adsorbent beds, in which there is periodically determined an absolute difference between the oxygen concentration in the nitrogen desorbed from one of the beds and the oxygen concentration in the nitrogen desorbed from the other bed, and there is periodically adjusted the duration and time of purging, so as to reduce said absolute difference, the period being defined relative to a predetermined duration or to a maximum concentration of oxygen in the nitrogen.
Furthermore, U.S. Pat. No. 5,407,465 teaches a PSA process comprising at least two adsorbent beds, which is regulated as a function of the determination of the variations of the temperature profile within the adsorbent beds; this process permits eliminating the problems of purging and of excessive or insufficient duration.
Furthermore, U.S. Pat. No. 5,258,056 proposes a PSA process whose regulation is carried out by determination of a reference signal and comparison of the latter with a predetermined value, such as to derive a valve control signal adapted to regulate the supply flow of gas entering the system.
However, none of these documents permits solving the problem imposed by variations of ambient temperature on the performance of a PSA unit; ambient air being, in the context of the present invention, the air contained within a building or an enclosure that can be heated or not, or external air, which is to say under atmospheric conditions, taken as such or if desired pretreated.
Thus, it is known that the suction temperature of the machine, which is to say the temperature of ambient air sucked in by the compressor supplying the adsorber or adsorbers, varies considerably as a function of the time of year, which is to say the season, of the geographic location in which the PSA unit is installed and, more generally, of the climate prevailing on site.
However, such intake temperature fluctuations give rise to important variations of the performance of the PSA unit in the course of the year, the latter being more or less degraded according to the ambient temperature.
A solution to overcome this problem of fluctuation of intake temperature could consist in controlling the supply temperature, which is to say the temperature of the air introduced to within the adsorbers, by placing for example one or more heat exchangers between the source of compression of air and the adsorbers, so as to reheat or, as the case may be, to cool the intake air already reheated during compression and thereby to ensure the introduction, within the adsorbers, of air at a controlled supply temperature.
However, it has been observed that the control of supply temperature alone, remains insufficient, given that the intake temperature of the air supplying the compressor affects the quantity of material, which is to say the quantity of gas introduced into the adsorbers and, as a result, the pressures prevailing in the adsorbers and, correspondingly, the performance of the PSA process overall.
In other words, the quantity of air taken in by the compressor, when the ambient temperature is below 0.degree. C., for example, is not equal to that taken in by the compressor when the ambient temperature is about 30.degree. C., for example, everything else being equal.